The walls of hot gas carrying high-temperature gas reactors, e.g. of combustion chambers in gas turbine plants, require thermal protection of their supporting structure against hot gas attack. The thermal protection can be provided, for example, by a hot gas lining upstream of the actual combustion chamber wall, e.g. in the form of a ceramic heat shield. A hot gas lining of this kind is generally made up of a number of metal or ceramic heat shield elements lining the surface of the combustion chamber wall. Because of their temperature resistance, corrosion resistance and low thermal conductivity, ceramic materials are ideally suited, compared to metal materials, for constructing a hot gas lining. A ceramic heat shield is described e.g. in EP 0 558 540 B1.
Because of the typical thermal expansion characteristics of the material and the temperature differences occurring during operation—e.g. between ambient temperature when the gas turbine plant is shut down and maximum full-load temperature—flexibility for thermal movement, particularly of ceramic heat shields, as a result of temperature-dependent expansion must be ensured, so that no heat-shield-destroying thermal stresses occur due to said temperature-dependent expansion being prevented. Expansion gaps are therefore provided between the individual heat shield elements in order to allow for thermal expansion of the heat shield elements. For safety reasons, the expansion gaps are designed such that they are never completely closed even at maximum hot gas temperature. It must therefore be ensured that the hot gas does not pass through the expansion gaps to the supporting wall structure of the combustion chamber. In order to seal the expansion gaps against the ingress of hot gas, they are frequently scavenged with seal air flowing in the direction of the combustion chamber interior. As seal air, air is generally used which is simultaneously employed as cooling air for cooling retaining elements for the heat shield elements, which results, among other things, in temperature gradients in the region of the edges of a heat shield element. As a result of the scavenging of the expansion gaps with seal air, the peripheral sides bordering the gaps as well as the cold side of the heat shield elements are cooled. On the other hand, a high heat input because of the hot gas takes place on the hot side of the heat shield elements. Inside a heat shield element, a three-dimensional temperature distribution therefore arises which is characterized by a temperature drop from the hot side to the cold side and by a temperature drop occurring from central points of the heat shield element toward the edges. Therefore, particularly in the case of ceramic heat shield elements, even without contact between adjacent heat shield elements, stresses occur on the hot side which may result in crack initiation and thus adversely affect the service life of the heat shield elements.
The heat shield elements in a gas turbine combustion chamber are typically of flat design and disposed parallel to the supporting structure. A temperature gradient running perpendicular to the surface of the supporting structure only results in comparatively low thermal stresses, as long as unhindered forward flexure in the direction of the interior of the combustion chamber is possible for the ceramic heat shield element in the installed state.
A temperature gradient running parallel to the supporting structure, such as that running from the peripheral surfaces of the heat shield element to the center of the heat shield element, quickly brings about increased thermal stresses because of the rigidity of plate-like geometries in respect of deformations parallel to their largest projection surface. These cause the cold edges of the peripheral surfaces, because of their comparatively low thermal expansion, to be placed under tension by hotter central regions which are subject to greater thermal expansion. If the material strength is exceeded, this tension can result in the initiation of cracks extending out from the edges of heat shield element toward central areas of the heat shield element.
The cracks reduce the load-bearing cross section of the heat shield element. The longer the cracks, the smaller the residual load-bearing cross section of the heat shield element. The thermally induced cracks may lengthen as the result of mechanical stress loads occurring during operation of the gas turbine plant, causing the residual cross section to be reduced still further and possibly necessitating replacement of the heat shield element. Mechanical stress loads of this kind may occur, for example, in the event of oscillatory accelerations of the combustion chamber wall which may be caused by combustion oscillations, i.e. oscillations in the combustion exhaust gases.
In order to reduce the seal air requirement—and therefore thermally induced stresses in heat shield elements, EP 1 302 723 A1 proposes providing flow barriers in the expansion gaps. This can also result in a reduction of the temperature gradient in the region of the edges. However, inserting flow barriers is not always easily possible and also increases the complexity of a heat shield.
In addition, heat shield elements are exposed to severe corrosive attack resulting in a lifetime-limiting loss of material. The material loss occurring in the case of ceramic heat shield elements is attributable to a combination of corrosion, subsequent resintering of the surface and erosive stress caused by the high mass flow of hot gas. Material loss is generally at its greatest where the highest hot gas flow rates obtain. For the ceramic heat shields frequently used nowadays made of corundum and mullite with glass phase, the material loss is essentially due to two reactions, namely first mullite decomposition and secondly grain growth and resintering. The water vapor present in the hot gas results in the decomposition of mullite (3Al2O3*2SiO, or 2Al2O3*1SiO2) and glass phase to corundum (Al7O3) and silicon oxide (SiOx) The corundum then present at the surface of a heat shield element, both in the matrix of the heat shield element and in the corrosion layer of the mullite grains, exhibits grain growth and sintering. Grain growth and sintering increase with operating time. With increasing numbers of gas turbine startups, this results in a weakening of the surface due to microcracking. Consequently, surface particles are entrained by the high mass flow, resulting in erosion. As a result, the service life of the heat shield elements is limited by corrosion, thereby necessitating premature replacement. Add to this the fact that, in the case of heavy oil operation of a gas turbine, magnesium oxide is added as an inhibitor, which likewise results in corrosive thinning of the heat shield elements. This is caused by the corundum in the heat shield element reacting with the magnesium oxide in the inhibitor to produce spinel as a reaction product. This also results in service life reduction and the need to replace the heat shield element prematurely.
DE 27 45 461 discloses a high refractory stone containing magnesium aluminate spinel (MgAl2O4), comprising 70 to 93 wt % magnesium aluminate spinel, 2 to 8 wt % aluminum oxide, 1 to 9 wt % binder and up to 27 wt % high refractory loading material. Loading materials specified are chromium(III) oxide (Cr2O3) and calcium zirconium oxide (CaZrO3). In addition, fused spinel, i.e. grains of fusion-cast spinel, are added in order to improve corrosion and thermal shock properties. DE2745461 describes low-CaO materials, but these still have a demonstrable SiO2 component.
DE 27 38 247 describes a high alumina refractory cement which can contain magnesium aluminate spinel. The addition of binders facilitates shaping of the high alumina cement. In addition, the products can be sintered for the first time during use, thereby saving firing costs. However, the disadvantage of such materials are the significantly poorer corrosion properties in a corrosive gas and/or fusion environment.
DE 102 54 676 A1 describes a refractory ceramic molded article with a structure comprising 80 to 95% zirconium oxide (ZrO2) and 5 to 20% magnesium aluminate spinel referred to the total weight, the addition of magnesium aluminate spinel being designed to produce higher thermal shock resistance.
Alternative approaches consist of using metal heat shield elements. Although metal heat shield elements are better able to withstand temperature fluctuations and mechanical stresses than ceramic heat shield elements, in gas turbine combustion chambers, for example, they require complex cooling of the heat shield, as they possess higher thermal conductivity than ceramic heat shield elements. Moreover, metal heat shield elements are more prone to corrosion and, because of their lower temperature stability, cannot be subjected to as high temperatures as ceramic heat shield elements.